Blade pile and method for increasing the bearing strength of pile

ABSTRACT

A pile and a method for increasing the bearing strength of a pile in which a pile having a helical, flexible blade is rotated or screwed into a pre-augered hole in the permafrost. Several embodiments are disclosed for reducing friction on the blade, by changing the shape of the blade or precutting a groove for the blade. Another embodiment of the method includes introducing a fill material between the surfaces of the blades and between the pile and the soil surrounding the pile and blades.

United States Patent Long [11 3,797,257 [45] Mar. 19, i974 4] BLADE PILEAND METHOD FoR INCREASING THE BEARING STRENGTH OF PILE Inventor: ErwinL. Long, 5741 College Dr.,

Anchorage, Alaska Filed: Aug. 4, 1972 Appl. No.: 278,048

[52] US. Cl. 61/535, 61/46 Int. Cl E02d 30/12, E02d 5/48 Field of Search61/53, 68, 35, 36, 46;

References Cited UNITED STATES PATENTS 3,646,766 3/1972 Hilton et al61/53.64

FOREIGN PATENTS OR APPLICATIONS 428,202 9/1926 Germany 61/535 849,4139/1960 Great Britain ..61/53 Primary Examiner-J. Karl Bell Pmmmumawu3797.257

sum 1 or 2 v IEJIGO 3 i BLADE PILE AND METHOD FOR INCREASING THE BEARINGSTRENGTH OF PILE BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to an improved pile for use in permafrost and toa method for increasing the bearing strength of pile in permafrost soil.

2. Description of the Prior Art v U.S. Pat. No. 3,217,791 discloses apile suitable for use in arctic soils which remain substantially frozenthroughout the year. My U.S. Pat. application, Ser. No. 1 14,258,entitled Method and Apparatus for Improving Bearing Strength of Piles inPermafrost, filed Feb. 10, 1971, now US. Pat. No. 3,706,204 discloses apile having rings secured to the lower portion of the pile. The pile isinserted into a hole with the ringed portion of the pile being confinedto the permanently frozen region of the hole. The hole is pre-augered toa diameter exceeding the diameter of the rings with the. interspacebetween pile and hole filled by a suitable slurry backflll. In someinstallations, the specialized type of equipment required to handle theslurry backfill is not available. In some cases, suitable materials forthe backfill are also either not available or because of sub-freezingtemperatures are difficult to store. As a result back filling techniqueshave limitations for use in many regions of the arctic.

SUMMARY OF THE INVENTION 7 It is an object of this invention to providea method of increasing the available bearing strength of permafrost soilby the use of a pile which is screwed into a pre-augered hole in thepermafrost soil.

It is another object of this invention to provide a method of improvingthe available bearing strength of permafrost soil at locations wherebackfill techniques are impracticah It is an object of this invention toprovide a unique pile which distributes the stress acting on the soilsurrounding the pile substantially uniformly throughout the length ofthe pile.

It is another object of this invention to provide a pile having aradially extending flexible spiral blade.

Basically these objects are obtained firstly by providing a pile havinga spiral blade which may with conventional equipment be screwed into ahole in the permafrost that is preaugered to a diameter less than themajor diameter of the blade. Secondly uniform distribution of the stressalong the length of the pile is obtained by using flexible membersextending outwardly from the pile so that limited strain is permitted atthe uppermost members without failing the surrounding soil. The flexiblemembers can be rings or segments of rings as in my earlier applicationSer. No. 114,258, a spiral blade as in the present invention, or anyother suitable radial extensions of the pile. Preferably, a combinedflexible spiral blade provides the maximum advantages.

BRIEF DESCRIPTION OF THE DRAWINGS F IG. 1 is a sectional view in apermafrost soil illustrat ing a preferred form of bladed pile embodyingthe principles of this invention.

FIG. 2 is a fragmentary section illustrating a preferred form of spiralblade.

FIGS. 3-5 illustrate alternate forms of cutting or forming blades topre'cut a groove in the soil.

FIG. 6ill'ustrates an alternative: form of blade.

FIG. 7 illustrates a typical stress strain curve for idealized frozensoil. 1

FIGS. 8 and 9 are charts showing a comparison of stress versus depth ofconventional pileand pile embodying the principles of this invention.

FIGS. 10A-10D are schematic illustrations of one method of installingbladed pile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates a typicalpile installation for use with this invention. A pile 10 which may besolid, hollow, primarily solid with an axial bore for permitting flow ofliquids to the bottom of the pile, or hollow pile of the type used toremove heat from the permanently frozen region of the soil, such as thethermopile illustrated in my prior patent No. 3,217,791 and shown hereinas the preferred embodiment. The thermopile illustrated is provided withlongitudinal heat distributing fins l2 and asealed internal pressurepipe or vessel 14 which contains a heat transferring liquid of a typedescribed in said patent. The pile is constructed to support a beam orother structural member 16. In accordance with the teachings of thisinvention, the pile is provided with spiral blade or fin 18. Preferablythe blade is limited to a lower portion of the pile which is insertedbelow the generally defined top level 19a of the permanently frozenregion 19 of the soil surrounding a pre-augered hole 20. As in saidearlier patent application, Ser. No. 114,258, the preferred form of piledoes not have a blade in the seasonal-thaw area above the permafrost sothat jacking forces due to thawing are reduced. The pre-augered orotherwise formed hole 20 is of a diameter greater than the diameter ofthe pile 10 but less than the major diameter of the blade 18 so that theblade 18 must cut into the soil surrounding the hole.

The blade 18 can be welded or otherwise secured to the pipe 14 along aspiral in a manner similar to a screw thread. Preferred designs rangefrom 4 gauge to as thin as 16 gauge, possibly even to 25 gauge. Therequired thickness of the blade, however, is partially controlled by theradial extent or height of the blade away from the pipe. The greaterthis dimension, the thicker the blade must be for a specificallydesigned unit stress. It is a unique feature of this invention, however,that some flexure occurs and is actually desired to improve the loadingon the pile. One of the basic problems in soil, and particularly infrozen soil, is the fact that very little strain of the frictionalsurface, i.e., the soil surface at which failure occurs, is required tobuild up a failure stress. This is best shown in FIG. 7. As illustratedin the stressstr'ain diagram, little movement of the surface of the pileis required to exceed the ultimate shearing stress of the soil to resultin failure. In fact, compres sion, that is reduction in length, of thepile under loading can actually exceed the strain required for failure.This is best illustratedin the diagrams of FIG. 8. Point U1 indicatesthe first ultimate failure point on a stress distribution curve duringinitial loading of the pile. As can be seen, it occurs at the uppermostregion of the soil. The stress distribution is indicated by the curve Elwith Y1 indicating the yield zone of the soil. It can thus be seen thatstress occurs on the soil surrounding the pile at its maximum near thetop of the pile and at its minimum a substantial distance up from thebottom of the pile. That is, the lowermost regions of the pile duringinitial loading of a conventional pile do not stress the surroundingsoil to any appreciable extent at all-rather all of the stress is borneby the soil near the top of the pile. After failure of the soil andsubsequent loading, the stress distribution occurs lower along the pileas indicated by curve E2, Y2, and U2. If the pile is again loaded to itsultimate at point U2, the soil will again fail. Subsequent loadings areindicated on curves E3, Y3, U3 and curves E4, Y4 and-U4 indicating ineach case that the stress distribution along the pile becomes loweralong the pile for each subsequent loading. As is apparent from thesecurves, the frictional resistance to resist failure of the soil islimited to a region less than the entire length of the pile,substantially reducing the bearing capacity of the pile. For thisreason, most of the strain during initial loading generally is assumedby the upper layers of the soil and in longer piles it is necessary tofail the upper layers of soil before the lower layers of soil can assumeany loading.

Using a flexible blade either of a spiral type as in this application,or non-spiral rings as in my prior application, Ser. No. 114,258, thepile is free to move downward as it is compressed with less movement ofthe tip of the blade so that the tip of the blade stays below the yieldstress of the soil to prevent failure. This means that the averageoverall stress of the total depth of the pile can be greatly increasedby obtaining a more uniform loading from the top to the bottom of thepile. Such a stress distribution for the flexible blade is illustratedin FIG. 9 which illustrates that the soil even at the bottom of theflexible bladed pile is subjected to stress during initial loading. Theflexure of the blade is, of course, limited to some extent by the frozensoil between the blade. Stated differently, the upper portions of thepile are still loaded first, as in conventional pile, however, in thebladed pile, this means only a temporary higher stress conditionfollowed by creep and strain of the soil in contact with and along sidethe blade then followed by bending of the blade. The maximum bending ofblade occurs at the upper limits of the pile and the least bending atthe lower limits of the pile. The degree offlexibility or stress straincharacteristics of the blade tip will be a function of the thickness ofthe blade, the height of the blade, and the type of material used.

The stress, height (radial) and strain characteristics of the blade canbe further modified by the use of a laterally tapered fin as illustratedin FIG. 6, as well as by the spacing of the levels of the blade. Therapidity with which the blade deforms might also be altered by thepresence of a void along the base of the blade such as where a slightoversized'hole is provided for the pile or pipe, as shown in FIG. 1, andby using water, which is more plastic, or a bentonite soil, whichfreezes gradually over considerable differences in temperature,providing a more plastic base around the inner ends of the blade. Stillanother alternative method is to pre-stress the rings downwardly by ashort term pre-loading for an even greater load capacity.

It is thus seen that a flexible blade, whether spiral or non-spiral,provides advantages over any type of pile used in permafrost soil. It isalso apparent that even a rigid spiral blade is uniquely advantageousfor use with non backfill application. With a spiral blade, as mentweenthe pile and the soil, this void can be filled with,

a water or slurry backfill if these materials are readily available. Onetechnique for applying the backfill is to extrude it under pressurethrough a center tube such as 24 so that the slurry is forced upwardsaround the pile 10 between the blade 18. An alternative technique is tofeed the fill material around the top of the pile between the blade atthe contact with the surface of the soil as the pile is being rotatedinto the ground so that the fill material fills the space between theblade as the pile is lowered. Both of these techniques are presentlyusable with existing equipment.

One of the limitations on this type of spiral bladed pile is the torquerequired to rotate or screw into the ground. For a given pile size andlength this torque is a function of friction. The friction is thefunction of the individual surface friction of any pile surface incontact with the soil and includes the blade surfaces, the angle of theblade, the degree of imbedment of the blade into the soil, the conditionof the soil through which the blade is cutting, and any additionallubrication that results from ice in soil, thawing, water or otherlubricant added. As the diameter of the blade gets larger, the torquefrom friction becomes more critical and there is a greater chance of themaximum torque exceeding that available from existing conventional pilerotating equipment.

Friction can be reduced by coating the surface of the pile and bladewith a lubricant. For example, the pile may have a hydrophobic typelubricating surface or a corrosion limiting surface which could beapplied with a material of lower friction value. On conventional piles,these types of low friction surfaces tend to reduce the bearingqualities of the pile and thus are a disadvantage. In the use of thebladed pile configuration, the low-friction coating does not reduce thebearing characteristics of the pile as the coatings are not along thesurface of potential failure which occurs at the tips of the blades.Thus, .by proper coating or other lubricating techniques, the torquerequirement can be reduced without limiting the bearing characteristicsof the pile.

1 warm seasons to thaw the permafrost and decrease friction. Heating toreduce friction of the permanently frozen region can also be applied byliquid carried down the pile through the center tube 24 while drilling.Vaporization of the thermopile fluids would then transfer heat to thecolder portions of the pile which would include the blade. This tends tothaw the permafrost in direct contact with this blade which reduces thefriction. Another technique is to apply heated air through the centertube allowing it to spiral up along the blade. The application ofheatedair or liquid through the center tube 24 has the added advantage ofdirectly heating the blade surfaces and at the leading lower bladeswhich do the initial cutting where friction reduction is most desired.In the case of a winter installation, if the unit were pre-pressurize das a thermopile there is a tendency for condensation to occur at the toprather than at the side. This can be countered in several ways duringinstallation: one is supplying heat down the center tube through thepressurizing fluid and two, by putting an insulated muff over theexposed portion of the pile.

Another technique for reducing friction is to use thicker blades such asthe forming blades 30, 32 and 34 in FIGS. 3 5, respectively, at thelower end of the pile to act as a pilot so that there is less drag onthe following spiral blade. In this technique the forming blades canhave a sharpened edge to increase cutting efficiency. FIG. 3 indicatesaforming blade 30 for cutting a groove substantially thicker than thepile blade 18. The blade is of substantially less height so that thetipof the pile blade still enters the soil in firm intimate contacttherewith. In FIG. 4 the forming blade 32 is tapered and is of the sameheight as the pile blade 18. The taper converges to a thickness lessthan the thickness of the pile blade at its radial tip. In FIG. 5 aforming blade 34 is curved to a thickness less than that of pile blade18 at the outer tip of the pile blade. The thicker forming blades at thelower end of the pile will act as a pilot so there will be less drag onthe following spiral blade, 18.

Alternately, in a particular problem condition, a special dummy pile canbe used with extra strong blade of slightly less height and greaterthickness and treated to have a lower frictional surface resistancewhere it might not otherwise be economically practical on the actualinstalled pile. Such a dummy pile is threaded into the hole and backedoff and then the actual pile to be used in construction is inserted andscrewed in the same groove with no actul loss in the bearing value.FIGS. A 10D illustrate a typical example of a method following thetechnique of using a dummy pile. In FIG. 10A a borehole 40 is firstangered to the desired depth. Next a dummy pile 42 is screwedinto thebore 40 cutting a spiral groove 41 of an outer diameter and thicknesscorresponding to one of the forming blade shapes shown in FIGS. 3 5.Next the dummy pile is backed off and removed from the bore and finallya pile is threaded down into the bore with the blade 18 engaged with thesoil in the grooves cut by the dummy pile.

While a preferred form of the apparatus and method for improving thebearing strength of permafrost soils has been illustrated and described,it should be understood that further alternative will be apparent tothose skilled in the art without departing from the principles of theinvention. Accordingly, the invention is not to be limited to thespecific techniques described but rather is to be limited only by aliteral interpretation of the claims appended hereto.

The embodiments of the invention in which an exclusive property orprivilage is claimed are defined as follows:

' l. A pile for use in soil having a permanently frozen region and aseasonal-thaw region comprising-a pile having a loading supporting upperend, means for securing a load bearing construction member to said upperend, said pile having a lower portion adapted to be placed in a hole insaid permanently frozen region and having an outer spiral screw bladethreadable into the soil surrounding said hole for increasing the shearstrength between the pile and the surrounding soil.

2. The pile of claim lsaid spiral screw having a base secured to saidpile and a radially outer relatively flexible. but stress resisting bodycapable of withstanding a substantial bending load prior to yielding andthen again withstanding a substantial further bending load afteryielding wherein the stress on the soil surrounding the hole duringloading is distributed along the entire length of the pile by allowingupper blades on the pile to deflect rather than shear the soilsurrounding those blades.

3. The pile of claim 1 said spiral blade having a leading cutting edgeat the bottom of the pile to reduce friction between the blade and thesurrounding soil whe screwing the pile into the soil.

4. The pile of claim 1 said blade having a frictionreducing coating.

5. The method of increasing the bearing strength of a pile in soilhaving a permanently frozen region and a seasonal-thaw region of thetype having an upper end secured to a load-bearing construction member,comprising providing a spiral blade on the pile on a lower portionthereof, forming a hole having a diameter greater than the diameter ofthe pile but less than the diameter of said spiral blade, and screwingsaid blade into the soil surrounding said hole whereby downward movementof said pile due to loads imposed on said construction member causesstress in the soil between and around said blade.

6. The method of claim 5 said step of screwing including the steps offirst cutting a spiral groove in the soil of a diameter greater than thehole diameter, and

second screwing the blade into the previously cut groove.

7. The method of claim 6 said blade being limited to the lower portionof the pile and said step of screwing including screwing the blade intothe permanently frozen region of the soilwith substantially all of theblade being below the seasonal thaw region. H

8. The method of claim 5 including the step of adding a fill material tothe hole as the blade is screwed into the ground.

9. The method of claim 7 wherein the fill material is added betweenadjacent blades as they are entering the hole.

10. The method of claim 7 wherein the fill material is forced up throughthe blade from the bottom of the hole.

'11. The method of claim 5 said screwing step including heating theblade as it is rotated in the hole.

12. The method of claim 5 said blade being relatively flexible butcapable of sustaining bending loads, and including the step of flexingthe upper end of the blade by loading the pile downwardly for causingdownward movement of the pile relative to the radially outer tip of theupper end of the blade to load the soil located axially downward of theupper end of the blade and thereby. increase the stress of the soilaround the lower end of the spiral blade.

13. The method of claim 12 including the step of pre stressing the bladeaxially downward.

14. A pile for use in soils having permafrost and seasonal-thaw regionscomprising a pile body having a load supporting upper end, means forsecuring a load bearing construction member to said upper end, said pilebody having a plurality of flexible appendages radially extending over asubstantial area from said body and enga'geable into said soil of saidpermafrost region, said flexible appendages being capable ofwithstanding a substantial bending load prior to yielding and then againwithstanding a substantial further bendingload sonal-thaw region andsaid side appendages being confined to said lower portion.

16. The pile of claim 14 said appendages being spirally arranged on saidpile body and including a forming blade at the lower end of the pilebody having a radially inner end for greater thickness along the lengthof the pile body then the remaining appendages but a radially outer endof less thickness along the length of the pile body than the remainingappendages at the outer end of the remaining appendages to act as apilot and reduce friction between the soil and the appendages when thepile body is screwed into the soil.

17. The method of increasing the bearing strenth of a load bearing pileof the type having an upper end secured to a load-bearing constructionmember and having a pile body and a plurality of load sustaining butflexible radially extending appendages secured to said pile bodycomprising forming a hole for receiving at least said pile body,inserting said pile body into said hole, and joining said appendagestosoil surrounding said pile body, loading said pile to effect downwardmovement of said pile body, and flexing upper appendages to allow atleast some further movement of said pile body without shearing thejoined soil.

18. The method of claim 17 said appendages including an elongatedcontinuous spiral blade, said step of forming a hole including formingthe diameter of the hole less than the diameter of said spiral blade,said step of inserting the pile body into the hole including screwingthe blade into the soil surrounding the hole.

19. The method of claim 18 said soil including a permanently frozenregion below a seasonal-thaw region, said blade terminating upwardlybelow said seasonalthaw region.

20. The method of claim 17 said hole having a diameter greater than saidappendages, said step of joining the appendages to soil surrounding saidpile body including filling the hole around and between said appendages.

21. The method of claim 20 said soil including a permanently frozenregion below a seasonal-thaw region, said blade terminating upwardlybelow said seasonalthaw region.

1. A pile for use in soil having a permanently frozen region and aseasonal-thaw region comprising a pile having a loading supporting upperend, means for securing a load bearing construction member to said upperend, said pile having a lower portion adapted to be placed in a hole insaid permanently frozen region and having an outer spiral screw bladethreadable into the soil surrounding said hole for increasing the shearstrength between the pile and the surrounding soil.
 2. The pile of claim1 said spiral screw having a base secured to said pile and a radiallyouter relatively flexible but stress resisting body capable ofwithstanding a substantial bending load prior to yielding and then againwithstanding a substantial further bending load after yielding whereinthe stress on the soil surrounding the hole during loading isdistributed along the entire length of the pile by allowing upper bladeson the pile to deflect rather than shear the soil surrounding thoseblades.
 3. The pile of claim 1 said spiral blade having a leadingcutting edge at the bottom of the pile to reduce friction between theblade and the surrounding soil when screwing the pile into the soil. 4.The pile of claim 1 said blade having a friction-reducing coating. 5.The method of increasing the bearing strength of a pile in soil having apermanently frozen region and a seasonal-thaw region of the type havingan upper end secured to a load-bearing construction member, comprisingproviding a spiral blade on the pile on a lower portion thereof, forminga hole having a diameter greater than the diameter of the pile but lessthan the diameter of said spiral blade, and screwing said blade into thesoil surrounding said hole whereby downward movement of said pile due toloads imposed on said construction member causes stress in the soilbetween and around said blade.
 6. The method of claim 5 said step ofscrewing including the steps of first cutting a spiral groove in thesoil of a diameter greater than the hole diameter, and second screwingthe blade into the previously cut groove.
 7. The method of claim 6 saidblade being limited to the lower portion of the pile and said step ofscrewing including screwing the blade into the permanently frozen regionof the soil with substantially all of the blade being beLow the seasonalthaw region.
 8. The method of claim 5 including the step of adding afill material to the hole as the blade is screwed into the ground. 9.The method of claim 7 wherein the fill material is added betweenadjacent blades as they are entering the hole.
 10. The method of claim 7wherein the fill material is forced up through the blade from the bottomof the hole.
 11. The method of claim 5 said screwing step includingheating the blade as it is rotated in the hole.
 12. The method of claim5 said blade being relatively flexible but capable of sustaining bendingloads, and including the step of flexing the upper end of the blade byloading the pile downwardly for causing downward movement of the pilerelative to the radially outer tip of the upper end of the blade to loadthe soil located axially downward of the upper end of the blade andthereby increase the stress of the soil around the lower end of thespiral blade.
 13. The method of claim 12 including the step ofprestressing the blade axially downward.
 14. A pile for use in soilshaving permafrost and seasonal-thaw regions comprising a pile bodyhaving a load supporting upper end, means for securing a load bearingconstruction member to said upper end, said pile body having a pluralityof flexible appendages radially extending over a substantial area fromsaid body and engageable into said soil of said permafrost region, saidflexible appendages being capable of withstanding a substantial bendingload prior to yielding and then again withstanding a substantial furtherbending load after yielding for allowing upper blades on the pile todeflect rather than shear the soil surrounding those upper blades todistribute the stress of the soil surrounding the pile body along moreof the length of the pile body.
 15. The pile of claim 14 said pile bodyhaving a lower portion insertable into the permafrost below saidseasonal-thaw region and said side appendages being confined to saidlower portion.
 16. The pile of claim 14 said appendages being spirallyarranged on said pile body and including a forming blade at the lowerend of the pile body having a radially inner end for greater thicknessalong the length of the pile body then the remaining appendages but aradially outer end of less thickness along the length of the pile bodythan the remaining appendages at the outer end of the remainingappendages to act as a pilot and reduce friction between the soil andthe appendages when the pile body is screwed into the soil.
 17. Themethod of increasing the bearing strenth of a load bearing pile of thetype having an upper end secured to a load-bearing construction memberand having a pile body and a plurality of load sustaining but flexibleradially extending appendages secured to said pile body comprisingforming a hole for receiving at least said pile body, inserting saidpile body into said hole, and joining said appendages to soilsurrounding said pile body, loading said pile to effect downwardmovement of said pile body, and flexing upper appendages to allow atleast some further movement of said pile body without shearing thejoined soil.
 18. The method of claim 17 said appendages including anelongated continuous spiral blade, said step of forming a hole includingforming the diameter of the hole less than the diameter of said spiralblade, said step of inserting the pile body into the hole includingscrewing the blade into the soil surrounding the hole.
 19. The method ofclaim 18 said soil including a permanently frozen region below aseasonal-thaw region, said blade terminating upwardly below saidseasonal-thaw region.
 20. The method of claim 17 said hole having adiameter greater than said appendages, said step of joining theappendages to soil surrounding said pile body including filling the holearound and between said appendages.
 21. The method of claim 20 said soilincluding a permanently frozen region below a seasonal-thaw region, saidblade terminating upwardly belOw said seasonal-thaw region.